As is well known, long power transmission lines are susceptible to instabilities in line frequency and voltage and also to transient overvoltages due to abrupt changes in operating conditions such as load drop-off.
Reactive power compensators are commonly used in such transmission lines to minimize transmission losses and to maintain the line voltage levels within prescribed limits. A common scheme of this type includes a shunt reactor located at a specified junction along the line as described, e.g., in A. Edlinger et al, "Use of High Voltage Chokes for Compensation of Very High Voltage Transmission Lines", CIGRE Report Number 402 (1964).
In order to provide stabilization in such types of systems, a capacitor may be serially connected in the line as described in the above-mentioned report. Alternatively, stabilization can be provided by means of a choke having DC-controlled bias magnetization and connectable directly to the line in the manner described, e.g., in H. Becker et al, "Three-Phase Shunt Reactors with Continuously Controlled Reactive Current, " CIGRE Report Number 31 - 13 (1972). Another type of stabilizer employs a nonlinear choke having compensation windings and associated filter circuitry, such choke being connected to the line via a conventional distributional transformer in the manner described, e.g., in E. Friedlander, "Static Network Stabilization," GEC Journal, pages 58 - 64 (1966).
Presently known compensators of these types have several disadvantages. The arrangements connectable directly to the line are unable to fully limit over-voltages during start-up conditions. The use of a conventional distribution transformer coupled between the line and the remainder of the compensator results in the excitation of harmonic and subharmonic oscillations because of the reactance of the transformer. Even the DC magnetization scheme, which avoids these immediate problems, is limited in its effectiveness because of the time constant of the DC circuit.